| 1. |
- Bjørgen, Johan P., et al.
(author)
-
Three-dimensional modeling of chromospheric spectral lines in a simulated active region
- 2019
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 631
-
Journal article (peer-reviewed)abstract
- Context. Because of the complex physics that governs the formation of chromospheric lines, interpretation of solar chromospheric observations is difficult. The origin and characteristics of many chromospheric features are, because of this, unresolved.Aims. We focus on studying two prominent features: long fibrils and flare ribbons. To model these features, we use a 3D magnetohydrodynamic simulation of an active region, which self-consistently reproduces both of these features.Methods. We modeled the Ha, Mg it k, Call K, and Call 8542 A lines using the 3D non-LTE radiative transfer code Multi3D. To obtain non-LTE electron densities, we solved the statistical equilibrium equations for hydrogen simultaneously with the charge conservation equation. We treated the Call K and Mg It k lines with partially coherent scattering.Results. This simulation reproduces long fibrils that span between the opposite -polarity sunspots and go up to 4 Mm in height. They can be traced in all lines owing to density corrugation. In contrast to previous studies, Ha, Mg II h&k, and Call H&K are formed at similar height in this model. Although some of the high fibrils are also visible in the Call 8542 A line, this line tends to sample loops and shocks lower in the chromosphere. Magnetic field lines are aligned with the Ha fibrils, but the latter holds to a lesser extent for the Call 8542 A line. The simulation shows structures in the Ha line core that look like flare ribbons. The emission in the ribbons is caused by a dense chromosphere and a transition region at high column mass. The ribbons are visible in all chromospheric lines, but least prominent in Call 8542 A line. In some pixels, broad asymmetric profiles with a single emission peak are produced similar to the profiles observed in flare ribbons. They are caused by a deep onset of the chromospheric temperature rise and large velocity gradients.Conclusions. The simulation produces long fibrils similar to what is seen in observations. It also produces structures similar to flare ribbons despite the lack of nonthermal electrons in the simulation. The latter suggests that thermal conduction might be a significant agent in transporting flare energy to the chromosphere in addition to nonthermal electrons.
|
|
| 2. |
- Cheung, Mark C. M., et al.
(author)
-
Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE). II. Flares and Eruptions
- 2022
-
In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 926:1
-
Journal article (peer-reviewed)abstract
- Current state-of-the-art spectrographs cannot resolve the fundamental spatial (subarcseconds) and temporal (less than a few tens of seconds) scales of the coronal dynamics of solar flares and eruptive phenomena. The highest-resolution coronal data to date are based on imaging, which is blind to many of the processes that drive coronal energetics and dynamics. As shown by the Interface Region Imaging Spectrograph for the low solar atmosphere, we need high-resolution spectroscopic measurements with simultaneous imaging to understand the dominant processes. In this paper: (1) we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne observatory to fill this observational gap by providing high-cadence (<20 s), subarcsecond-resolution spectroscopic rasters over an active region size of the solar transition region and corona; (2) using advanced numerical models, we demonstrate the unique diagnostic capabilities of MUSE for exploring solar coronal dynamics and for constraining and discriminating models of solar flares and eruptions; (3) we discuss the key contributions MUSE would make in addressing the science objectives of the Next Generation Solar Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope (and other ground-based observatories) can operate as a distributed implementation of the NGSPM. This is a companion paper to De Pontieu et al., which focuses on investigating coronal heating with MUSE.
|
|
| 3. |
- da Silva Santos, João Manuel, 1992-, et al.
(author)
-
Heating of the solar chromosphere through current dissipation
- 2022
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 661
-
Journal article (peer-reviewed)abstract
- Context. The solar chromosphere is heated to temperatures higher than predicted by radiative equilibrium. This excess heating is greater in active regions where the magnetic field is stronger.Aims. We aim to investigate the magnetic topology associated with an area of enhanced millimeter (mm) brightness temperatures in a solar active region mapped by the Atacama Large Millimeter/submillimeter Array (ALMA) using spectropolarimetric co-observations with the 1-m Swedish Solar Telescope (SST).Methods. We used Milne–Eddington inversions, nonlocal thermodynamic equilibrium (non-LTE) inversions, and a magnetohydrostatic extrapolation to obtain constraints on the three-dimensional (3D) stratification of temperature, magnetic field, and radiative energy losses. We compared the observations to a snapshot of a magnetohydrodynamics simulation and investigate the formation of the thermal continuum at 3 mm using contribution functions.Results. We find enhanced heating rates in the upper chromosphere of up to ∼5 kW m−2, where small-scale emerging loops interact with the overlying magnetic canopy leading to current sheets as shown by the magnetic field extrapolation. Our estimates are about a factor of two higher than canonical values, but they are limited by the ALMA spatial resolution (∼1.2″). Band 3 brightness temperatures reach about ∼104 K in the region, and the transverse magnetic field strength inferred from the non-LTE inversions is on the order of ∼500 G in the chromosphere.Conclusions. We are able to quantitatively reproduce many of the observed features including the integrated radiative losses in our numerical simulation. We conclude that the heating is caused by dissipation in current sheets. However, the simulation shows a complex stratification in the flux emergence region where distinct layers may contribute significantly to the emission in the mm continuum.
|
|
| 4. |
- Danilovic, Sanja
(author)
-
Modeling of chromospheric features and dynamics in solar plage
- 2023
-
In: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 71:4, s. 1939-1947
-
Journal article (peer-reviewed)abstract
- The chromosphere is a dynamic and complex layer where all the relevant physical processes happen on very small spatio-temporal scales. A few spectral lines that can be used as chromospheric diagnostics give us convoluted information that is hard to interpret without realistic theoretical models. What are the key ingredients that these models need to contain? The magnetic field has a paramount effect on chromospheric structuring. This is obvious from the ubiquitous presence of chromospheric dynamic fibrilar structures visible on the solar disk and at the limb. The numerical experiments presented in this manuscript illustrate the present state of modeling. They showcase to what extent our models reproduce various chromospheric features and their dynamics. The publication describes the effect different ingredients have on chromospheric models and provides a recipe for building one-to-one models. Combining these models with observations will provide insight into the physical processes that take place in the solar atmosphere.
|
|
| 5. |
- Danilovic, Sanja, et al.
(author)
-
Rapid blue- and redshifted excursions in Hα line profiles synthesized from realistic 3D magnetohydrodynamic simulations
- 2023
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 670
-
Journal article (peer-reviewed)abstract
- Context. Rapid blue- and redshifted excursions (RBEs and RREs) may play an important role in mass-loading and heating the solar corona, but their nature and origin are still debatable.Aims. We aim to model these features to learn more about their properties, formation, and origin.Methods. We created a realistic three-dimensional (3D) magnetohydrodynamic model of a solar plage region. Synthetic Hα spectra were generated and the spectral signatures of these features identified. The magnetic field lines associated with these events were traced, and the underlying dynamic was studied.Results. The model reproduces many properties of RBEs and RREs well, such as spatial distribution, lateral movement, length, and lifetimes. Synthetic Hα line profiles, similarly to observed ones, show a strong blue- or redshift as well as asymmetries. These line profiles are caused by the vertical component of velocities higher than 30 − 40 km s−1, which mostly appear in the height range 2 − 4 Mm. By tracing magnetic field lines, we show that the vertical velocity that causes the appearance of RBEs or RREs is always associated with the component of velocity perpendicular to the magnetic field lines.Conclusions. The study confirms the hypothesis that RBEs and RREs are signs of Alfvénic waves with, in some cases, a significant contribution from slow magneto-acoustic modes.
|
|
| 6. |
- de la Cruz Rodriguez, Jaime, et al.
(author)
-
STiC : A multiatom non-LTE PRD inversion code for full-Stokes solar observations
- 2019
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 623
-
Journal article (peer-reviewed)abstract
- The inference of the underlying state of the plasma in the solar chromosphere remains extremely challenging because of the nonlocal character of the observed radiation and plasma conditions in this layer. Inversion methods allow us to derive a model atmosphere that can reproduce the observed spectra by undertaking several physical assumptions. The most advanced approaches involve a depth-stratified model atmosphere described by temperature, line-of-sight velocity, turbulent velocity, the three components of the magntic field vector, and gas and electron pressure. The parameters of the radiative transfer equation are computed from a solid ground of physical principles. In order to apply these techniques to spectral lines that sample the chromosphere, nonlocal thermodynamical equilibrium effects must be included in the calculations. We developed a new inversion code STiC (STockholm inversion Code) to study spectral lines that sample the upper chromosphere. The code is based on the RH forward synthesis code, which we modified to make the inversions faster and more stable. For the first time, STiC facilitates the processing of lines from multiple atoms in non-LTE, also including partial redistribution effects (PRD) in angle and frequency of scattered photons. Furthermore, we include a regularization strategy that allows for model atmospheres with a complex depth stratification, without introducing artifacts in the reconstructed physical parameters, which are usually manifested in the form of oscillatory behavior. This approach takes steps toward a node-less inversion, in which the value of the physical parameters at each grid point can be considered a free parameter. In this paper we discuss the implementation of the aforementioned techniques, the description of the model atmosphere, and the optimizations that we applied to the code. We carry out some numerical experiments to show the performance of the code and the regularization techniques that we implemented. We made STiC publicly available to the community.
|
|
| 7. |
- De Pontieu, Bart, et al.
(author)
-
Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE). I. Coronal Heating
- 2022
-
In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 926:1
-
Journal article (peer-reviewed)abstract
- The Multi-slit Solar Explorer (MUSE) is a proposed mission composed of a multislit extreme ultraviolet (EUV) spectrograph (in three spectral bands around 171 Å, 284 Å, and 108 Å) and an EUV context imager (in two passbands around 195 Å and 304 Å). MUSE will provide unprecedented spectral and imaging diagnostics of the solar corona at high spatial (≤05) and temporal resolution (down to ∼0.5 s for sit-and-stare observations), thanks to its innovative multislit design. By obtaining spectra in four bright EUV lines (Fe ix 171 Å, Fe xv 284 Å, Fe xix–Fe xxi 108 Å) covering a wide range of transition regions and coronal temperatures along 37 slits simultaneously, MUSE will, for the first time, "freeze" (at a cadence as short as 10 s) with a spectroscopic raster the evolution of the dynamic coronal plasma over a wide range of scales: from the spatial scales on which energy is released (≤05) to the large-scale (∼170'' × 170'') atmospheric response. We use numerical modeling to showcase how MUSE will constrain the properties of the solar atmosphere on spatiotemporal scales (≤05, ≤20 s) and the large field of view on which state-of-the-art models of the physical processes that drive coronal heating, flares, and coronal mass ejections (CMEs) make distinguishing and testable predictions. We describe the synergy between MUSE, the single-slit, high-resolution Solar-C EUVST spectrograph, and ground-based observatories (DKIST and others), and the critical role MUSE plays because of the multiscale nature of the physical processes involved. In this first paper, we focus on coronal heating mechanisms. An accompanying paper focuses on flares and CMEs.
|
|
| 8. |
- Diaz Baso, Carlos José, et al.
(author)
-
Solar image denoising with convolutional neural networks
- 2019
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 629
-
Journal article (peer-reviewed)abstract
- The topology and dynamics of the solar chromosphere are greatly affected by the presence of magnetic fields. The magnetic field can be inferred by analyzing polarimetric observations of spectral lines. Polarimetric signals induced by chromospheric magnetic fields are, however, particularly weak, and in most cases very close to the detection limit of current instrumentation. Because of this, there are only few observational studies that have successfully reconstructed the three components of the magnetic field vector in the chromosphere. Traditionally, the signal-to-noise ratio of observations has been improved by performing time-averages or spatial averages, but in both cases, some information is lost. More advanced techniques, like principal-component analysis, have also been employed to take advantage of the sparsity of the observations in the spectral direction. In the present study, we use the spatial coherence of the observations to reduce the noise using deep-learning techniques. We designed a neural network that is capable of recovering weak signals under a complex noise corruption (including instrumental artifacts and non-linear post-processing). The training of the network is carried out without a priori knowledge of the clean signals, or an explicit statistical characterization of the noise or other corruption. We only use the same observations as our generative model. The performance of this method is demonstrated on both synthetic experiments and real data. We show examples of the improvement in typical signals obtained in current telescopes such as the Swedish 1 m Solar Telescope. The presented method can recover weak signals equally well no matter what spectral line or spectral sampling is used. It is especially suitable for cases when the wavelength sampling is scarce.
|
|
| 9. |
- Kianfar, Sepideh, et al.
(author)
-
Physical properties of bright Ca II K fibrils in the solar chromosphere
- 2020
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 637
-
Journal article (peer-reviewed)abstract
- Context. Broad-band images of the solar chromosphere in the Ca II H&K line cores around active regions are covered with fine bright elongated structures called bright fibrils. The mechanisms that form these structures and cause them to appear bright are still unknown.Aims. We aim to investigate the physical properties, such as temperature, line-of-sight velocity, and microturbulence, in the atmosphere that produces bright fibrils and to compare those to the properties of their surrounding atmosphere.Methods. We used simultaneous observations of a plage region in Fe I 6301-2 Å, Ca II 8542 Å, Ca II K, and Hα acquired by the CRISP and CHROMIS instruments on the Swedish 1 m Solar Telescope. We manually selected a sample of 282 Ca II K bright fibrils. We compared the appearance of the fibrils in our sample to the Ca II 8542 Å and Hα data. We performed non-local thermodynamic equilibrium inversions using the inversion code STiC on the Fe I 6301-2 Å, Ca II 8542 Å, and Ca II K lines to infer the physical properties of the atmosphere.Results. The line profiles in bright fibrils have a higher intensity in their K-2 peaks compared to profiles formed in the surrounding atmosphere. The inversion results show that the atmosphere in fibrils is on average -100 K hotter at an optical depth log (τ500 nm) = -4.3 compared to their surroundings. The line-of-sight velocity at chromospheric heights in the fibrils does not show any preference towards upflows or downflows. The microturbulence in the fibrils is on average 0.5 km s(-1) higher compared to their surroundings. Our results suggest that the fibrils have a limited extent in height, and they should be viewed as hot threads pervading the chromosphere.
|
|
| 10. |
- Leenaarts, Jorrit, et al.
(author)
-
Chromospheric heating during flux emergence in the solar atmosphere
- 2018
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 612
-
Journal article (peer-reviewed)abstract
- Context. The radiative losses in the solar chromosphere vary from 4 kW m(-2) in the quiet Sun, to 20 kW m(-2) in active regions. The mechanisms that transport non-thermal energy to and deposit it in the chromosphere are still not understood. Aims. We aim to investigate the atmospheric structure and heating of the solar chromosphere in an emerging flux region. Methods. We have used observations taken with the CHROMIS and CRISP instruments on the Swedish 1-m Solar Telescope in the Ca II K, Ca II 854.2 nm, H alpha, and Fe I 630.1 nm and 630.2 nm lines. We analysed the various line profiles and in addition perform multi-line, multi-species, non-local thermodynamic equilibrium (non-LTE) inversions to estimate the spatial and temporal variation of the chromospheric structure. Results. We investigate which spectral features of Ca II K contribute to the frequency-integrated Ca II K brightness, which we use as a tracer of chromospheric radiative losses. The majority of the radiative losses are not associated with localised high-Ca II K-brightness events, but instead with a more gentle, spatially extended, and persistent heating. The frequency-integrated Ca II K brightness correlates strongly with the total linear polarization in the Ca II 854.2 nm, while the Ca II K profile shapes indicate that the bulk of the radiative losses occur in the lower chromosphere. Non-LTE inversions indicate a transition from heating concentrated around photospheric magnetic elements below log tau(500) = -3 to a more space-filling and time-persistent heating above log tau(500) = -4. The inferred gas temperature at log tau(500) = -3.8 correlates strongly with the total linear polarization in the Ca II 854.2 nm line, suggesting that that the heating rate correlates with the strength of the horizontal magnetic field in the low chromosphere.
|
|
